1. Field of the Invention
The present invention relates to a stimulating hearing prosthesis, and more particularly, to using multi-electrode channel configurations in a stimulating hearing prosthesis.
2. Description of the Related Art
To compensate for a deficiency, such as a loss of hearing, several electrical stimulation devices use electrical signals to activate nerve or muscle fibers in a recipient. In particular, a cochlear implant directly stimulates functioning auditory nerves inside the cochlea with electric fields stimulated through electric current pulses. These implant devices typically include a microphone that receives incoming sound and a signal processor that converts selected portions of the incoming sound into corresponding stimulating signals based on a selected sound processing strategy. An internal receiver implanted beneath the skin receives the signals and delivers electric current pulses to an array of electrodes coiled inside the cochlea. The electrodes stimulate the auditory nerve fibers in the cochlea and the resulting electrical sound information is carried along the auditory nerve to the brain for interpretation. Each electrode may provide monopolar or bipolar stimulation. Monopolar stimulation is stimulation delivered from a single intracochlear electrode to a remote extracochlear electrode. Bipolar stimulation occurs when stimulation flows from nearby, paired intracochlear electrodes. Bipolar stimulation focuses the stimulation more and presumably stimulates a smaller, more localized population of auditory nerve fibers. Monopolar stimulation, on the other hand, spreads current over a relatively wider area and stimulates a relatively larger population of neurons.
Using monopolar stimulation, traditional cochlear implants cannot focus stimulation on target neurons that are electrically distant from the stimulating electrodes. For example, in implant devices which use electrodes placed along the length of the scala tympani to stimulate the spiral ganglion cells, there is an inability to focally stimulate small subpopulations of spiral ganglion cells with monopolar stimulation of the placed electrodes. Because the bone surrounding the scalae has an impedance that is relatively higher than the impedance of the perilymph and tissue that fill the scalae, stimulation currents tend to spread longitudinally along the length of the cochlea. Longitudinal current spread results in relatively broad neural excitation patterns compared to those elicited by narrowband acoustic stimulation in healthy ears. Attempts to narrow the broad neural excitation patterns caused by monopolar stimulation have been made using channel configurations with two or more nearby electrodes to source and sink all or some of the current. But, when multiple channels are stimulated simultaneously with the same polarity, electric fields accumulate and neural stimulation patterns combine in a non-linear fashion. While channel configurations of multiple electrodes may result in more focused patterns, there is still usually some significant channel interactions between nearby channels. To avoid the adverse effects of channel interactions, which tend to be greatest with monopolar channel configurations, most traditional implant devices use stimulation strategies that incorporate sequential or temporally “interleaved” stimulation patterns.